In millimeter wave imaging systems a scene is scanned in order to obtain an image of the scene. In many imaging systems the antenna is mechanically moved to scan over the scene. However, electronic scanning, i.e. electronically moving the radiation beam or the sensitivity profile of the antenna, is preferred as it is more rapid and no deterioration of the antenna occurs like in a mechanic scanning system.
Reflectarray antennas are a well-known antenna technology, e.g. as described in J. Huang et J. A. Encinar, Reflectarray Antennas, New York, N.Y., USA: Institute of Electrical and Electronics Engineers, IEEE Press, 2008, used for beam steering in the microwave and millimeter waves frequency range (hereinafter commonly referred to as “microwave frequency range” covering a frequency range from at least 1 GHz to 30 THz, i.e. including mm-wave frequencies). For frequencies up to 30 GHz there exist multiple technologies to control the phase of each individual antenna element of such a reflectarray antenna having different advantages and disadvantages. In particular PIN diode based switches suffer from a high power consumption, high losses and can hardly be integrated into a microwave antenna operating above 100 GHz. MEMS switches require high control voltages and have very slow switching speed. FET-based switches suffer from high insertion losses and require a large biasing network. Liquid crystal based phase shifters exhibit very slow switching speeds in the order of tenths of a second. Ferroelectric phase shifters allow rapid shifting at low power consumption, but have a significant increase in loss above 60 GHz.
Optically controlled plasmonic reflectarray antennas are described, for instance, in U.S. Pat. No. 6,621,459 and M. Hajian et al., “Electromagnetic Analysis of Beam-Scanning Antenna at Millimeter-Waves Band Based on Photoconductivity Using Fresnel-Zone-Plate Technique”, IEEE Antennas and Propagation Magazine, Vol. 45, No. 5, October 2003. Such reflectarray antennas have, however, a very high power consumption. Particularly, U.S. Pat. No. 6,621,459 discloses a plasma controlled millimeter wave or microwave antenna in which a plasma of electrons and holes is photo-injected into a photoconducting wafer. In a first embodiment the semiconductor is switched between the material states “dielectric” and “conductor” requiring a high light intensity and providing a high antenna efficiency. In a second embodiment the semiconductor is switched between the two states “dielectric” and “absorber (lossy conductor)” requiring only a low light intensity and providing a worse antenna efficiency. A special distribution of plasma and a millimeter wave/microwave reflecting surface behind the wafer allows a phase shift of the individual elements of 180° between optically illuminated and non-illuminated elements in the first embodiment. The antenna can be operated at low light intensities using a mm-wave/microwave reflecting back surface with an arbitrary constant phase shift between illuminated and non-illuminated elements in said second embodiment.
In an embodiment the antenna includes a controllable light source including a plurality of LEDs arranged in an array and a millimeter wave reflector positioned in front of the light source, said reflector allowing light from the light source to pass there through while serving to reflect incident millimeter wave radiation. Further, an FZP (Fresnel Zone Plate) wafer is positioned in front of the millimeter wave reflector, said wafer being made a photoconducting material which is transmissive in the dark to millimeter waves and is responsive in the light. Finally, the antenna includes an antenna feed located in front of the wafer for illuminating the wafer with millimeter waves and/or receiving millimeter waves. By selectively illuminating the LEDs, heavy plasma density produces a 180° phase shift in out-of-phase zones. With respect to those regions where the LEDs are not illuminated, low plasma density (or “in-phase”) zones are provided. Millimeter wave radiation which is incident on the high plasma density zones incurs a 180° phase change on reflection at the front surface of the wafer. Comparatively, millimeter wave radiation which is incident on the low plasma density zones incurs a 180° phase change on reflection at the millimeter wave reflector. The path length difference provides the desired overall phase shift of 180° between in-phase and out-of-phase zones. In an alternative embodiment described in this document the reflectivity of the wafer to reflect millimeter wave radiation is changed by the illumination of the light source to either allow the millimeter wave radiation to be reflected or to pass through. In another embodiment using lower light intensities the mm-wave radiation can either be absorbed by the wafer or pass through.